“Sally” made a comment on my recent Post “Unidentified Fossil Stone from Slope Point Polished” that is very intriguing. I am grateful for her suggestion. There are some tiny shapes in this stone that have been very difficult to identify. Some of them look like long-stemmed mushrooms (as circled below).
One suggestion is that the stone is fossilised forest floor, something that occurs in the Slope Point area. I had wondered at one point if they might be fossil sponge spicules. Sally commented as follows:
The tiny long-stemmed fungi resemble Rayleigh-Taylor instability. This concept is used to describe various fluid dynamical systems, including when a heavier fluid is suspended above a lighter fluid within a gravitational field. I wonder if it can approximate cooling lava as it forms solid rock. See this link for images of Rayleigh-Taylor simulations: https://computing.llnl.gov/projects/blast/rayleigh-taylor-instability. I’m going to guess this pebble is/was orbicular rhyolite, an extrusive igneous rock formed by silica-rich volcanic ash. I’d love to hear from a geologist about it. Very beautiful!
Wikipedia notes: The Rayleigh–Taylor instability… is an instability of an interface between two fluids of different densities which occurs when the lighter fluid is pushing the heavier fluid. Examples include the behaviour of water suspended above oil while under the influence of Earth’s gravity, and mushroom clouds like those from volcanic eruptions and atmospheric nuclear explosions. One publication illustrates the way the two fluids interact, over time and at a certain temperature, as follows:
The fourth and fifth stages here produce a shape very similar to the tiny “fossils” observed in my Slope Point find.
Sally suggested the stone could have started off as orbicular rhyolite – the very similar sphericular rhyolite is often found at Slope Point, though it is relatively rare.
Sally’s suggestion that the tiny long-stemmed fungi shapes could be due to Rayleigh-Taylor instability may or may not be correct, and it will be difficult to verify it. However, it is an intriguing hypothesis that is not out of the bounds of possibility.
